The present invention relates to turbine engines. In particular, the present invention relates to devices for attaching ceramic turbine shrouds to surrounding metallic turbine components.
A gas turbine engine commonly includes a fan, a compressor, a combustor, a turbine, and an exhaust nozzle. During engine operation, working medium gases, for example air, are drawn into and compressed in the compressor. The compressed air is channeled to the combustor where fuel is added to the air and the air/fuel mixture is ignited. The products of combustion are discharged to the turbine section, which extracts work from these products to produce useful thrust to power, for example, an aircraft in flight. A portion of the work extracted from the products of combustion by the turbine is used to power the compressor.
The compressor and turbine commonly include alternating stages of rotor blades and stator vanes. Compressor and turbine rotors include stationary annular fluid seals surrounding the blades and acting to contain and direct the flow of working medium fluid through successive stages. The annular compressor and turbine rotor seals, sometimes referred to as turbine shrouds and outer air seals, may be attached to the engine by, for example, a support case.
The operating temperatures of some engine stages, such as in the high pressure turbine stages, may exceed the material limits of the metallic turbine shroud and therefore necessitate cooling the shroud by using, for example, compressor bleed air directed to the segment through the support rings. Ceramic materials have been studied for application to components in the hot section of gas turbine engines to replace metallic materials that require such cooling in order to withstand the high temperature of combustion gas. For example, ceramics have been employed in the fabrication of hybrid turbine rotors and integrally bladed rotors (IBRs), which are sometimes referred to as bladed disks or simply blisks. In both cases, particularly that of a ceramic IBR, a large gap between rotor blade tip and metal shrouds commonly results from the low thermal expansion of ceramics that make up the blades and the IBRs. The low density and high stiffness of ceramics reduce the radial displacement of the blade tip and therefore exacerbates the issue further. The large gap or clearance at the blade tip results in a high percentage of hot working medium gas flow leaking through the tip-shroud gap that reduces the transfer of energy from the gas flow to turbine blades, which in turn causes an engine performance penalty as useful energy is not harnessed. The performance penalty is more severe for small gas turbine engines because the engine size makes a small tip clearance large relative to the gas flow path.
To minimize losses induced by large tip clearance, ceramic shrouds have been employed to control the gap between rotor blade tip and the inner surface of the shroud. Due to its high stiffness, low thermal expansion and high thermal conductivity, a ceramic shroud experiences less thermal distortion than a metal shroud for a given set of thermal loading conditions. The high temperature capability of the ceramics also leads to reduced cooling air requirement, which provides an additional benefit to engine performance by reducing the amount of energy that must be diverted from propulsion to cooling.
The main difficulty in ceramic shroud design is the attachment to the metallic engine structure because of low ductility and low thermal expansion of ceramics relative to metals. Elastic springs have been used to support ceramic shrouds, but their performance at elevated temperatures over long durations is questionable due to metal creep. Another common technique of supporting a ceramic shroud is through a tab and slot approach in which tabs on the ceramic shroud engage slots on a metallic casing. Generally there are a number of tab and slot pairs evenly distributed circumferentially to spread the support load and to position the shroud radially. This method is directed at minimizing thermal constraints by allowing the ceramic shroud and metal support to grow independent of each other. However, in practice, due to manufacturing tolerance control, uneven thermal fields, and thermal deformation of the shroud and the casing, thermal stress at the tabs may be sufficiently high to cause local damage. Finally, shrink-fitting a metallic support to the ceramic shroud provides the advantage of introducing compressive stress into the ceramic shroud and improves shroud reliability. However, it is difficult to control the clamp load from the shrink-fit metallic support on the shroud over a wide range of thermal transient conditions.